Following the flurry of operational weather modification programs in India over the last decade, the Indian Institute of Tropical Meteorology (IITM) implemented a multi–year program to study cloud–aerosol interactions and cloud seeding. An Indian research program called "Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX)" was implemented in 2008. CAIPEEX is a national program with participation from different research and governmental organizations in India. During Phase I in May to September 2009 aerosol and cloud microphysics observations were collected over different meteorological regimes and at different locations over India. In 2010, Phase II was conducted during an intensive observation period (IOP) from August to November.

CAIPEEX Phase I observations provided an opportunity to study the variability in the aerosol and cloud microphysical properties over the rainy regions (Western Ghats) and rain shadow regions to the east. One of the significant results showed that warm rain initiation starts at early stages (lower cloud depths) over the coastal areas and becomes even lower at the crest of the western slopes, probably due to washout of aerosols by precipitation. Clouds lose their microphysically maritime properties farther east over the rain shadow regions, where the clouds have to grow to higher depths to initiate the precipitation process.

CAIPEEX Phase II explored the sensitivities of clouds to aerosols and the thermodynamic environment in which they develop. It is becoming apparent that aerosol loading may not be the only factor that determines the cloud microphysical properties of clouds. Other factors such as the thermodynamic properties in the rain shadow region when compared to coastal areas may play a more important role in the differences observed in cloud microphysics and precipitation. Phase II also produced measurements of cloud microphysical properties needed to study the physical plausibility that hygroscopic cloud seeding could have produced a change in a "seeded" cloud segment when compared to a "non–seeded" cloud segment. Sufficient data was collected by experienced scientists, including scientists at RAL, which flew the aircraft precisely to target the seeded cloud volume.

The research work was lead by IITM and its execution was supported by NCAR's RAL, Woodley Weather Consultants (WWC), Orsmond Aviation and University of the Witwatersrand (WITS). Local logistics were supported by Krishi Consultants.

Project Summary

The objective of CAIPEEX–II is to examine the variability of thermodynamic properties, aerosol, clouds, and precipitation through airborne and radar measurements over the central region of India during the monsoon period. The goals of the IOP were to:

1. Document these conditions and determine the suitability of clouds to cloud seeding and to determine the optimal methods to target clouds
2. Measure effects of cloud seeding on cloud microstructure

The approach in measuring seeding effect was to recognize seeding opportunities, implement a seeding strategy, and evaluate the effects of seeding. This was done in an attempt to establish a cause–and–effect relationship between the seeding action and the changes in the cloud microphysical properties measured by the aircraft and radar.

In order to accomplish these objectives various flight strategies where considered, including:

1. Instrumentation tests and intercomparisons. These operations involved intercomparison of duplicate instruments (such as comparing the two FSSPs and the two PCASPs) and flying in close spatial coordination with rawinsondes to compare aircraft state variables with sounding measurements.
2. Coordinated cloud physics and seeding research flights required to explicitly test and evaluate hypotheses regarding cloud precipitation processes, seeding hypotheses and seeding trials. These flight operations utilized the research aircraft and the seeding aircraft operating in close spatial and temporal coordination. The research aircraft measured cloud processes in seeded and non–seeded cloud segments; the treated cloud volumes were sprayed with SF6 gas, which was released concurrently with the hygroscopic material by the seeder aircraft at the time of seeding. This was done in an attempt to identify seeded cloud segments unambiguously.
3. Ambient aerosol research survey flights required to document and understand the aerosol content of the lower atmosphere under a variety of synoptic and regional weather conditions.
4. Cloud and aerosol research flights required to document and understand the microstructure of clouds and the effectiveness of natural precipitation processes. These studies focused on convective clouds in non–precipitating and precipitating form.
5. Radiation measurements required for measurements of upwelling and downwelling radiative fluxes. This required the aircraft to fly at constant straight–and–level legs in a stepped sequence.